infra/common: document the use of --depth parameter for repo init

There have been reports of sync failure specifically for linux kernel.
In order to workaround such a failure if it occurs, the use of --depth
parameter for the repo init command has been documented. With depth set
to 1 (--depth=1), the commit history that is download is reduced to just
one commit and this reduces the overall sync time and reduces failure to
sync the linux kernel source.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I726e20124804b02514172cdee27a132c3acb22dd